How do I check if a firebase database value exists?

I have some suggestions by using firebase.You check it from firebase.

We can test for the existence of certain keys within a DataSnapshot using its exists() method:

A DataSnapshot contains data from a Firebase database location. Any time you read data from a Firebase database, you receive the data as a DataSnapshot.

A DataSnapshot is passed to the event callbacks you attach with on() or once(). You can extract the contents of the snapshot as a JavaScript object by calling its val() method. Alternatively, you can traverse into the snapshot by calling child() to return child snapshots (which you could then call val() on).

A DataSnapshot is an efficiently-generated, immutable copy of the data at a database location. They cannot be modified and will never change. To modify data, you always use a Firebase reference directly.

exists() - Returns true if this DataSnapshot contains any data. It is slightly more efficient than using snapshot.val() !== null.

Example from firebase documentation(javascript example)

var ref = new Firebase("https://docs-examples.firebaseio.com/samplechat/users/fred");
ref.once("value", function(snapshot) {
  var a = snapshot.exists();
  // a === true

  var b = snapshot.child("rooms").exists();
  // b === true

  var c = snapshot.child("rooms/room1").exists();
  // c === true

  var d = snapshot.child("rooms/room0").exists();
  // d === false (because there is no "rooms/room0" child in the data snapshot)
}); 

Also please refer this page(already mentioned in my comment)

Here there is an example using java.

Firebase userRef= new Firebase(USERS_LOCATION);
userRef.child(userId).addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        if (snapshot.getValue() !== null) {
            //user exists, do something
        } else {
            //user does not exist, do something else
        }
    }
    @Override
    public void onCancelled(FirebaseError arg0) {
    }
});

I hope you got an idea now.

You can check snapshot.exists value.

NSString *roomId = @"room1";
FIRDatabaseReference *refUniqRoom = [[[[FIRDatabase database] reference]
                                      child:@"rooms"]
                                     child:roomId];

[refUniqRoom observeSingleEventOfType:FIRDataEventTypeValue
                            withBlock:^(FIRDataSnapshot * _Nonnull snapshot) {

    bool isExists = snapshot.exists;
    NSLog(@"%d", isExists);
}];

self.ref = FIRDatabase.database().reference()

   ref.child("rooms").observeSingleEvent(of: .value, with: { (snapshot) in

        if snapshot.hasChild("room1"){

            print("true rooms exist")

        }else{

            print("false room doesn't exist")
        }


    })

While the answer of @ismael33 works, it downloads all the rooms to check if room1 exists.

The following code accomplishes the same, but then only downloads rooms/room1 to do so:

ref = FIRDatabase.database().reference()

ref.child("rooms/room1").observeSingleEvent(of: .value, with: { (snapshot) in
    if snapshot.exists(){
        print("true rooms exist")
    }else{
        print("false room doesn't exist")
    }
})